Preheat oven to 425°F (220°C).
Heat a cast iron skillet over medium heat.
Add butter and let it sizzle.
Add diced onions and cook for 5–6 minutes, stirring often.
Stir in jalapeños and cook for 2 minutes more.
Add chorizo, breaking it apart as it cooks.
Increase heat to medium-high if needed.
Cook chorizo for 6–7 minutes, or until fully browned.
Transfer cooked chorizo to a paper-towel-lined plate to drain excess grease.
Return drained chorizo to the skillet, reserving a small amount for garnish.
Begin adding the cheeses to the skillet.
Let the cheese melt for 1–2 minutes.
Fill a roasting pan halfway with hot water.
Place the skillet into the water bath.
Transfer the pan to the oven.
Bake for 18–22 minutes, or until cheese is fully melted and gooey.
Remove from oven and garnish with reserved chorizo.
Top with fresh cilantro.
Serve immediately with tortillas or tortilla chips.